1. Field of the Invention
This invention relates to balancing traffic loads and improving throughput in network communication, and more particularly to a QoS (quality of service) based load-balancing scheme for multiple-band access points (APs), wireless local are network (WLAN) switches, switched multiple APs, and clustered AP centralized management or distributed but synchronized management.
2. Description of Related Art
Wireless Local Area Networks (WLAN) have become a popular option to wired LANs, especially at locations where wiring is difficult or costly. Conventional wired LANs are typically geographically limited. Although a single access point (“AP” hereinafter) can support a relatively large group of network stations, it functions only within a finite range of typically several hundred feet. Extended coverage areas can be accomplished by installing multiple access points with overlapping coverage cells, so that network stations can roam throughout the area without ever losing network contact. A typical wireless LAN can use up to hundreds of access points, and thus the cost of the access points can strongly influence the cost of the entire system.
In order to provide transparent connectivity between the computers on the wired LAN and the network stations, an access point processes all packets on its backbone interface. Access points usually lock at the destination address of each data packet, and consult internal tables to determine whether the packet should be received and forwarded out its wireless interface.
Referring to FIG. 1, a schematic overview of a wireless LAN 10 including several access points AP1, AP2 and AP3, each of which has its own coverage areas 20, 30 and 40. Many network stations are present of which 50 and 60 are shown. In this method, the decision of a network station 50 to switch from an access point AP1 to another access point AP2, or AP3 for load balancing is dependent on communication quality of each respective access point and the traffic load of each access point and of the network station itself. The access points AP1, AP2, and AP3 monitor their traffic load preferably, by keeping record of the average TX/RX (transmission/reception) rate activity time value averaged over a certain time interval. On the other hand, when the communication equality decreases below a predetermined level, the network station 50 starts to search for a client 30, 40 (an access point AP2, AP3) with a better communication quality. As known from IBEE 802.11, the other access points AP2, AP3 may be operating on channels with other frequencies than the associated access points AP1. For maintaining better communication quality for the network stations, such as 50 and 60, installing multiple access points to obtain extended coverage areas with overlapping coverage cells is a good solution. However, some specific network station may be covered by several access points. Load balancing over multiple access points (APs) is proposed in the art.
Load balancing over multiple access points (APs) can only be found in a few commercially available wireless local area network (WLAN) switch implementations. In those cases, either the number of connections or the level of utilization is used to benchmark the traffic conditions for deciding whether load-balancing measurement needs to be activated. In all of these cases, traffic is treated as single class and the difference between traffic type and their priority level is not taken into account.
Referring to one conventional scheme of load balancing for wireless LAN, which refers to a European Patent Application EP 156623A1, published on Nov. 21, 2001, titled “Wireless LAN with Load Balancing” proposed by Murray Hill, et al., of Lucent Technologies Inc. In the proposed scheme of load balancing, a communication system with a plurality of access points and at least one network station is provided. For the load balancing purpose, the system selects a communication connection with one of the access points using a predetermined cost function. The predetermined cost function taking the access point traffic load parameters and the network station traffic load parameters into account. In this scheme, a client collects traffic and reception information forming the access points providing radio coverage. The client then uses a cost function to decide the best access point for association. This scheme has three shortcomings. Firstly, all clients underlying hardware and firmware design are manufacturer-dependent. In many cases, finite state machines in hardware are used to handle packet association, which renders this conventional method impractical to implement. Secondly, this conventional scheme adopts reception as the parameter in the cost functions. In prioritized or multimedia traffics with guaranteed quality of service (“QoS”, hereinafter), bandwidths available under certain reception level may not necessarily match to the traffic type of the client that looks for an appreciate access point to associate. Third, the assumption of this proposed scheme based on a single subnet for all covering access points. In reality, multiple subnets may be involved in a physical network of access points. The access point advertises a right reception level and traffic conditions may not have be of a right virtual local area network (VLAN). For example, a client wants to associate with the VLAN for a financial department, but that VLAN's corresponding SSID (Service Set Identifier) is not available in the access point that gives the optimal result in cost function.
Another conventional load balancing is provided in U.S. Pat. No. 5,987,062, issued on Nov. 16, 1999, titled “Seamless Roaming For Wireless Local Area Networks” by Darwin A. Engwer, et at of Netwave Technologies, Inc. In the proposed architecture, a wireless LAN allows roaming of a network station to allow it to serially associate with a number of access points of the network fixed backbone. This roaming is supported by an improved measurement of communications link quality, which includes calculating a mean error free length of a test pattern, which is a digital data message, broadcast by each access point and received by a network station. Thus an accurate measurement of link quality is provided which allows the network station to determine whether it should change its association to another access point having improved communications link quality. Further, a load balancing process is provided to balance the communications load amongst a variety of access points, by allowing network stations also to switch their association with access points in accordance with a current total data rate at any given access point and also considering the number of currently high data rate network stations associated with a particular access point at any one time. This scheme has some shortcomings. One of them is that all clients underlying hardware and firmware design are manufacturer-dependent. Each network station within range should have the function to receive the test pattern broadcast by each access point and compare it to an identical test pattern previously stored in the network station. In addition, a beacon searching process, which is supported by a separate process called scanning, accomplishes the load balancing process proposed in the architecture. The purpose of scanning is to supply the information to keep current each network station's AP list. In scanning, the network station periodically tunes to the various hop frequencies and listens for a short beacon from any AP. This tuning is out of the hop sequence of the AP with which that MU is currently registered. Upon receipt of a short Beacon, the network station enters the short beacon data in its AP list. In the architecture, roaming is supported by software (e.g. firmware) which is a set of computer programs partially located in each AP and partially located in each network station, which cause poor compatibility for the network station.
Yet another conventional method in prior art is proposed in an International Patent Application WO 01/43467, published on Jun. 14, 2001, titled “Flexible Wireless LAN Architecture Based On A Communication Server” by Juan Grau, et al. of Proxim, Inc. In the architecture, a wireless LAN system includes a wireless communication server and one or more access points operably connected to the wireless communication server. The access points are adapted to wirelessly transmit and receive data to and from remote network stations using radio frequency communications such that the remote network stations form part of a wireless LAN. The wireless communication server is physically separate from the access points. The wireless communication server maintains centralized filtering and forwarding of data to be transmitted to the remote units. In the architecture, a method of directing data to a remote network station in a wireless LAN is also proposed. In the wireless communication server, network data is analyzed to determine, from the remote network station identification, a desired access point to transmit the data. The wireless communication server is adapted to select the desired access point from a number of possible access points. In the wireless communication server, the data is redirected to the correct access point. In an access point, data is wirelessly transmitted to the remote network station using a radio frequency communication link. The load balancing method is proposed by using a centralized load-balancing communication server to manage a set of access points. However, this work does not go into specifics of the load-balancing policies and the nature of the traffic types.